1. Field
The present patent document relates generally to a method for manufacturing MRAM devices, and, more particularly, to a method for manufacturing MTJ pillars for MTJ memory devices with improved density and quality specifications.
2. Description of the Related Art
Magnetoresistive random-access memory (“MRAM”) is a non-volatile memory technology that stores data through magnetic storage elements. These elements are two ferromagnetic plates or electrodes that can hold a magnetic field and are separated by a non-magnetic material (i.e., a barrier layer), such as a non-magnetic metal or insulator. In general, one of the plates has its magnetization pinned (i.e., a “reference layer”), meaning that this layer has a higher coercivity than the other layer and requires a larger magnetic field or spin-polarized current to change the orientation of its magnetization. The second plate is typically referred to as the free layer and its magnetization direction can be changed by a smaller magnetic field or spin-polarized current relative to the reference layer.
MRAM devices store information by changing the orientation of the magnetization of the free layer. In particular, based on whether the free layer is in a parallel or anti-parallel alignment relative to the reference layer, either a “1” or a “0” can be stored in each MRAM cell. Due to the spin-polarized electron tunneling effect, the electrical resistance of the cell changes due to the orientation of the magnetic fields of the two layers. The cell's resistance will be different for the parallel and anti-parallel states and thus the cell's resistance can be used to distinguish between a “1” and a “0”. One important feature of MRAM devices is that they are non-volatile memory devices, since they maintain the information even when the power is off.
MRAM devices are considered to be the next generation structures for a wide range of memory applications. The magnetic tunnel junction (“MTJ”) layer stack and processing of the MTJ layer stack into pillars for MTJ memory devices are the two most critical aspects of the MRAM technology development. However, under conventional manufacturing schemes, forming pillar like MTJ devices without shunts and at a DRAM-like density is not manufacturable.
One limitation of the current processing technology is illustrated in FIG. 1. Once the photomask and hard mask are formed, the MTJ stack is etched using directional ion beams 110. During the etching process, the material that is removed from the base of the MTJ pillar 120 is re-deposited on the side of the MTJ pillar. This re-deposited material 130 contains metals such as iridium (Ir), platinum (Pt), ruthenium (Ru) metals and that do not form insulating oxides. As a result, their presence at the edges 122 of the barrier layer of the MTJ pillar 120 is significantly detrimental to the operation of the device. Specifically, this conductive re-deposited material 130 shorts the barrier and renders the tunnel junction of the MTJ structure inoperable. Conventional manufacturing processes alleviate this problem by performing side cleaning at very high ion beam angles (usually 70°) to remove the re-deposited material 130 on the sides 122 of the barrier layer of the MTJ pillar. However, this removal process puts unacceptable limits on the device density. For example, as shown in FIG. 1, for an MTJ device structure of 100 nm, the ion beam cleaning requires spacing of approximately 270 nm, which is significantly larger than the density requirements of 100 nm or less between adjacent MTJ pillars. Furthermore, sidewall cleaning at high ion beam angles significantly increases the beam damage to the thin MTJ layers, which only further compromises the MTJ performance. Although the MRAM development companies have spent significant resources and effort to develop ion beam cleaning techniques as well as other manufacturing processes, such as reactive ion etching, the existing manufacturing processes have not lead to a satisfactory process and tooling technology for MTJ pillars.
Accordingly, there is a strong felt need for a manufacturing method for MTJ pillars for a MTJ memory device that meets density and quality requirements for future MTJ memory product application.